Certain well known mowers have one or more grass cutting units operatively connected to a traction vehicle such that the motion of the traction vehicle propels the cutting units over the ground to cut grass. The cutting units can comprise reel cutting units with each cutting unit having a reel that rotates about a horizontal axis to sweep the grass against a sharpened bedknife to cut the grass by severing the grass between the reel and the bedknife. Alternatively, the cutting units can comprise rotary cutting decks with each cutting unit having a blade that rotates about a vertical axis to cut the grass by impact of the sharpened cutting edges of the blade with the grass. When the mower carries a plurality of cutting units, the cutting units are usually spaced and staggered relative to one another in some fashion to collectively cut a swath of grass that is wider than the width of any individual cutting unit. Such a mower is often referred to as a gang mower. The Toro Groundsmaster 4500-D is an example of a gang mower that carries a plurality of rotary cutting decks.
The cutting units of such a mower are connected to the traction vehicle by lift arms that are moved by hydraulic cylinders to lift the cutting units off the ground to place them into a transport position or to lower the cutting units into engagement with the ground to place them into a mowing position. When the cutting units are so lowered into engagement with the ground, each cutting unit is substantially self supporting on the ground and rolls over the ground by virtue of various rotatable members, such as rollers, that are carried on each cutting unit and that engage the ground. The cutting units, the lift arms, and the various connections therebetween and with the traction vehicle are designed to allow each cutting unit to pivot with various degrees of freedom to allow each cutting unit to pitch, and/or roll, and/or yaw to conform itself to the contours of the ground in a free floating manner. When the cutting units are down in the mowing position and in this free floating cutting mode, the hydraulic cylinders that act on the lift arms of the cutting units are also typically in a float mode where the piston rod is free to move back and forth within each cylinder. This same result can also be achieved using a locked cylinder position and a lost motion slot in the lift arm.
It has long been known in the mower art that such self supporting, rolling cutting units transfer most of their weight directly to the ground when the cutting units are down and in their mowing position. It has also long been known in the mower art that this can adversely affect the traction of the traction vehicle. It has further been known that there are times when it would be desirable to transfer at least a portion of the weight of the cutting units back to the traction vehicle to put more downforce on the wheels of the traction vehicle and thereby increase the driving effectiveness or traction of the wheels. This weight transfer has often been accomplished in the mower art by some type of counterbalance system that tends to reduce how much of the cutting units weight is supported by the ground. This effectively puts more of the weight of the cutting units on the traction vehicle and less of their weight on the ground, though the weight transfer is never so great as to lift the cutting units completely off the ground since the cutting units must maintain contact with the ground in the mowing position.
One type of known prior art counterbalance system is shown in U.S. Pat. No. 4,864,805 to Hager et al., assigned to The Toro Company, the assignee of this invention. In Hager, the counterbalance system establishes one of two selected backpressures in the hydraulic fluid return circuit from lift and lower cylinders 14, namely either a 400 psi backpressure or a 600 psi backpressure as determined by which one of two different pressure relief valves is effective at a particular time. Either backpressure tends to bias the cutting units upwardly to transfer some of the weight of the cutting unit back to the traction vehicle, but the 400 psi backpressure will transfer less weight back to the traction vehicle than will the 600 psi backpressure. Thus, in the counterbalance system shown in Hager, the 400 psi backpressure is the normal or default backpressure that is present in the hydraulic fluid return circuit and thus at least some of the weight of the cutting units is always being transferred back to the traction vehicle.
As the mower shown in Hager is being operated, there will be times with the pressure in the hydraulic drive circuit to the hydraulic drive motors for the drive wheels will increase, e.g. when the traction vehicle is climbing a hill and the traction demands on the vehicle increase. A pilot line ports the hydraulic pressure in the drive motor circuit to a pilot operated hydraulic directional valve that in one position selects the 400 psi pressure relief valve and in the other position selects the 600 psi pressure relief valve. When the hydraulic pressure in the drive motor circuit increases enough, the on-off hydraulic valve shifts over and selects the 600 psi backpressure for use, thus transferring more weight from the cutting units to the traction vehicle in an attempt to maintain traction.
While the Hager counterbalance system is effective, it provides only two preselected values for the backpressure, namely either 400 psi or 600 psi. In certain commercial embodiments of the Hager counterbalance system, the two preselected values for the backpressure were closer together, namely 620 psi and 700 psi than the values described in the Hager patent. However, in either case, when the higher backpressure was suddenly applied instead of the lower backpressure, the abrupt nature of the shift in traction on the traction vehicle could often be felt or sensed by the operator on the traction vehicle. Such an abrupt traction shift could potentially be distracting or alarming to the operator, particularly to an unskilled operator, and thus represented a disadvantage of the prior art system. In addition, if the traction vehicle lost traction before the counterbalance system applied the higher backpressure, the counterbalance system was not designed to allow the traction vehicle to regain traction once traction was lost.